------- Comment #18 from ramana at gcc dot gnu dot org 2010-02-10 23:45 ------- (In reply to comment #16) > In fwprop.c of r118475, we get to propagate_rtx_1 (fwprop.c:334): > > /* Copy propagations are always ok. Otherwise check the costs. */ > if (!(REG_P (old) && REG_P (new)) > && !should_replace_address (op0, new_op0, GET_MODE (x))) > return true; > > At this point the simplified address has been found, but fwprop decides not to > substitute the new address: > > (gdb) p debug_rtx(op0) > (plus:SI (reg/f:SI 102) > (const_int 4 [0x4])) > $58 = void > (gdb) p debug_rtx(new_op0) > (plus:SI (reg/f:SI 25 sfp) > (const_int -8 [0xfffffffffffffff8])) > $59 = void > (gdb) p should_replace_address(op0,new_op0,SImode) > $60 = 0 '\000' > > The replacement isn't done because fwprop sees no benefit in doing the > transformation. Stepping through should_replace_address we get: > > 202 gain = address_cost (old, mode) - address_cost (new, mode); > (gdb) next > 208 if (gain == 0) > (gdb) p gain > $64 = 0 > (gdb) next > 209 gain = rtx_cost (new, SET) - rtx_cost (old, SET); > (gdb) > 211 return (gain > 0); > (gdb) p gain > $65 = 0 > > Perhaps we should prefer addresses based on the frame pointer over other > addresses? >
Why does this sound like some bit of the discussion in this thread here ?http://gcc.gnu.org/ml/gcc/2009-12/msg00347.html -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39871
